1. Field of the Invention
The present invention relates to the removal of solvents from a solution to produce non-volatile material or precipitates, and more particularly to provide high rates of evaporation and sublimation commonly applied in compound processing procedures to accelerate production of dry non-volatile materials.
2. Background Information
Compound processing to separate mixtures of chemical compounds into pure individual components usually includes a series of sequential dissolution process steps interspersed with drying steps. The drying step may only increase the concentration of a liquid or solid, but often complete drying of the non-volatile precipitates is needed before a re-dissolution is performed. To recover the dissolved non-volatile materials as dry powders or to increase the concentration of compounds, three approaches are used: a) vacuum centrifuges; b) freeze drying; and c) blow down concentrators. Multi-step compound processing is time consuming and expensive, and drying steps are in the critical path of any process because drying must be complete before the next step begins. Any reduction in drying time is advantageous.
Vacuum centrifuges combine a high performance vacuum pump to create very low pressure conditions inside the chamber of a centrifuge rotor to increase the rate at which solvent molecules can escape from the surface of a solution. The centrifugal force keeps solutions in their containers and prevents the violent boiling of the solution in the vacuum environment. A cold trap is usually configured to scavenge solvent molecules as they migrate from the higher concentration space inside the vacuum centrifuge to the low concentration space inside a solvent collection vessel in the cold trap. High performance vacuum systems are needed to speed drying, but such vacuum systems are expensive to buy and operate. However, these expensive, high performance systems working with water-containing solvent (which is the most commonly occurring solvent in biological samples) still only have the ability to remove water at about 0.5 mL/hr. It would be advantageous to increase the removal rate of water in such systems.
Freeze drying is a technique chosen typically when solute molecules are subject to degradation at temperatures above freezing or when in liquid solutions. This process requires that the solution be frozen initially. The container with the frozen solvent material is placed in the freeze drying apparatus. A hard vacuum is pulled on the surface of solid solution whereupon solvent molecules escape (sublime) from the solid solution. Since sublimation is endothermic it tends to maintain the solid frozen state of the solution. The now free solvent molecules migrate to and are collected in the lower concentration region of the cold trap solvent collection container. This approach retains the expensive vacuum system while adding, in some applications, expensive refrigeration equipment. If water is used as a solvent, the time to pre-freeze compound solutions adds to the time and effort to bring the compounds to dryness.
Blow-down concentrators create a continuous flow of a gas onto the surface of the liquid (or solid) solution. The gas flow promotes the escape of solvent molecules from the solution container so that they can be carried away in the flow of used gas out an exhaust port. The blow-down unit can be located inside a fume hood so that solvent vapors are not released to the workspace, or the exhaust port can be connected to a cold trap to capture the solvent molecules in a container. In every case, the exhaust outlet is directed to a fume hood. Blow-down concentrators are configured with specific gasses which are non-reactive to the solute compounds so that compound degradation does not occur. Nitrogen is commonly chosen to prevent degradation of potentially reactive compounds from oxygen in the air.
Blow down concentrators are not recommended for complete drying since the flow of drying gas will carry away the dry material. Using industry standard tube (test tube-like) tube containers, known blow down driers suggest a maximum of two (2) liters per minute of gas flow per solution tube container. Furthermore, it is suggested that the solution not be dried to a powder, but to a more concentrated liquid state, since this conservative gas flow rate may disturb a dry sample. The inability to dry completely a sample solution reduces the performance of blow down concentrators, and their consumption of gas (usually nitrogen) adds to the operating costs of blow down systems.